Certain industrial applications require AC line voltage sensing for operation and control. However, under certain scenarios, AC line voltage can be extremely dangerous especially to a user or operator of the system. One possible method of protecting users from a potential electric shock is to galvanically isolate the AC source (e.g., AC line voltage) from the control circuit that monitors the AC line voltage. The AC Line voltage can be very high, for example on the order of 500 Vrms or higher. Therefore, the monitored line voltage is preferably adapted (i.e., the monitored line voltage is preferably scaled down or reduced) for use with a low voltage monitoring circuit, such as a low voltage monitoring circuit that comprises semiconductor components.
A typical method of isolating and reducing a primary AC line voltage is by way of a step-down isolation transformer. The use of such isolation transformers presents both advantages as well as disadvantages. For example, one advantage of such an isolation method is that that low frequency transformers (50/60 Hz) are typically quite linear and relatively stable. However, these low frequency transformers are also generally large, heavy and expensive, especially for high input voltages such as voltages on the order of approximately 500 Vrms. Those transformers are not used as power transformers, because the voltage-monitoring device (as will be explained with respect to FIG. 1a) draws a relatively insignificant amount of current. For example, such a voltage-monitoring device may draw current on the order of a few micro-amps.
FIG. 1a illustrates a first typical voltage sensing circuit 10. Sensing circuit 10 comprises a high input voltage 12, a first typical isolation step-down transformer 14, and a voltage monitoring device 18. Isolation step-down transformer 14 steps down or reduces high input voltage 12 to a lower voltage or a scaled voltage 16. This scaled down voltage 16 may then be safely applied to voltage monitoring device 18.
Hence, the isolation transformers are deemed to be small, because they typically require just a few milli-watts of power under normal operating conditions. Nevertheless, under certain practical applications, isolation transformers can become rather large. For example, in certain industrial applications where 50/60 Hz three-phase power lines are used, these lines carry up to 680 Vrms and perhaps even higher voltages in certain applications. Despite relatively low transformer wattage requirements, the primary winding of a step-down transformer requires thousands of turns to meet a minimum required ratio “Turns per Volts”. This can be seen where the number of primary turns N of a transformer is represented by the following equation:N=(Vin·108)/(4.44ƒAB)                 where N=Number of primary turns; Vin=Primary Voltage; f=line-frequency; A=core cross-section Area; and, B=max. flux, in lines. In other words, the number of primary turns is proportional to the input voltage (E) and is in inversely proportion to the relative size of transformer (A). Consequently, a smaller transformer will require more turns for the same voltage, as well, a higher voltage requires more turns. Thousands of turns take additional space, resulting in these types of step-down transformers being heavy, generally expensive, and requiring a relatively large footprint.        
One alternative to using an isolation step-down transformer, such as the transformer illustrated in FIG. 1a, is to provide a voltage divider network. Such a voltage divider network provides a reduced or a low scaled voltage to the voltage monitoring device. One arrangement of such a voltage divider based voltage sensing circuit 20 is illustrated in FIG. 1b. A typical isolation transformer (such as transformer 14 of FIG. 1(a)) may be replaced with a miniature low voltage isolation transformer, if input voltage 22 is reduced by voltage divider such that a reduced voltage is applied across a miniature isolation transformer 26. For example, in one arrangement, an exemplary miniature isolation transformer will have a primary inductance of approximately (≈1-2 mH) and have a relatively small footprint. Such a footprint will be on the order of approximately 0.25 sq.in. or less.
Voltage sensing circuit 20 illustrated in FIG. 1b includes a high voltage input 22, a resistive voltage divider 24, and a miniature isolation transformer 26. Resistive voltage divider 24 is coupled in series to miniature isolation transformer 26 to reduce high voltage input 22. In one arrangement, miniature isolation transformer 26 comprises a miniature surface mount isolation transformer. One such miniature isolation transformer is manufactured by HALO, Electronics, Inc. of Mountainview, Calif. and bears Part Number: TGR-3755NC. Such a miniature isolation transformer has a primary inductance of L≈1 mH.
The input impedance of miniature transformers at a frequency of ƒ=50 Hz is relatively low and can be computed based on the following:Z=2*π*ƒ*L=2*3.14*50*(1*10−3)=0.314 Ohm where Z is the transformer impedance, ƒ is the signal frequency, and L is the transformer inductance. For accuracy, resistive voltage divider R 24 shall be selected to dissipate a minimum amount of power, thereby reducing voltage divider self-heating. In one arrangement, the power dissipation is chosen so as not to exceed 0.25 Watts. If it is assumed that the high voltage input 22 is chosen to equal 680 Vrms and the resistor power dissipation is 0.25 Watts, the voltage divider power may be calculated as follows:   P  =            V      2        R  Where V is the input line voltage and R is the divider resistance. So, in order not to exceed a power dissipation of 0.25 Watts, the resistance R shall be at least:       R    ≦                  V        2            P        =                    680        2                    0.25        ⁢                                   ⁢        W              ≈          2      ⁢                           ⁢      M      ⁢                           ⁢      Ohm      And the current of the miniature isolation transformer 26 may then be calculated as follows:   I  =            V      R        =                            680          ⁢                                           ⁢          V                          2          ⁢                                           ⁢          M          ⁢                                           ⁢          Ohm                    ≈              340        ⁢        μ        ⁢                                   ⁢        A            The input voltage applied to the miniature isolation transformer 26 is:V=Z*I=0.314 Ohm*340 μA≈107 microvolts The signal magnitude (V≈107 microvolts) is comparable to the magnitude of typical industrial electrical or electromagnetic interference. Therefore, in this scenario, if the voltage signal's magnitude is generally equal to a noise magnitude, the resulting Signal/Noise ratio approaches unity. Consequently, since it is generally known that an acceptable “Signal/Noise Ratio” is generally on the order of 40-120 db (100-1000 times), a Signal/Noise Ratio=1 is generally unacceptable.
The precision of measurement is generally poor and unreliable. If the noise magnitude is essentially equal to the signal magnitude, the measurement error is 50%. Therefore, a better Signal/Noise Ratio will reduce the measurement error. This poor signal/noise ratio therefore makes the resistive voltage divider 24 difficult to use for a scale (reduce) the line voltage. Therefore, a low inductance miniature transformer cannot be used at the industrial frequency (i.e., 50/60 Hz) as an isolation device since its impedance is too low to provide a significant voltage level (Signal/Noise ratio) for the accurate measurement by voltage sensing device 30.
From the previous discussion, it can be noted that the linearity and the isolation properties of transformers are generally predictable. Transformers can provide precision scaling for measurements and they are preferable devices for isolation in most typical cases. However, the size, footprint, weight and cost of these transformers complicate their usage for voltage monitoring, especially when the size of the overall voltage sensing circuit tends to be critical.
There is, therefore, a general need for a voltage sensing technique that overcomes the foregoing concerns. There is a further need for a voltage sensing circuit that provides high voltage monitoring by implementing a miniature, cost effective isolation transformer while also producing acceptable and accurate voltage sensing measurements.